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SE-292: High Performance Computing

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Presentation on theme: "SE-292: High Performance Computing"— Presentation transcript:

1 SE-292: High Performance Computing
Memory Organization R. Govindarajan L1

2 Use of Main Memory by a Program
Instructions (code, text) Data used in different ways Stack allocated Heap allocated Statically allocated text data Heap Stack L7 Use of memory addresses

3 Stack Allocated Variables
Space allocated on function call, reclaimed on return Addresses calculated and used by compiler, relative to the top of stack, or some other base register associated with the stack Growth of stack area is thus managed by the program, as generated by the compiler Lec8

4 Heap Allocated Variables
Managed by a memory allocation library Functions like malloc, realloc ,free Get `linked’ (joined) to your program if they are called Executed just like other program functions What about growth of the heap area? Managed by the library functions Lec8

5 Memory Management: Protection
There could be many programs running on the same machine concurrently Sharing the resources of the computer Processor time, Main memory Must be protected from each other: one program should not be able to access the variables of another Typically done through Memory management schemes – paged memory, segmentation, … Involves Address Translation Lec8

6 Idea of Virtual Memory Each executing program uses addresses in the range 0 .. MaxAddress These addresses are not real, but only Virtual Addresses They are translated into real main memory addresses Many programs in execution can safely share main memory Terminology virtual address, physical address Lec8

7 Example: Main Memory Management
Example: Paged Virtual Memory To translate a virtual address to the corresponding physical address, a table of translation information is needed Minimize size of table by not managing translations on byte basis but larger granularity Page: fixed size unit of memory (contiguous memory locations) for which a single piece of translation information is maintained L9

8 … … … Paged Virtual Memory PROGRAM/PROCESS MAIN MEMORY
Virtual Address Space Physical Address Space 0x 0x Text 0x000000 0x000024: 256 Bytes 256 Bytes Virtual Page 0 Physical Page 0 0x000000FF 0x000000FF 0x0000FF 0x 0x Data 0x : 256 Bytes Virtual Page 1 0x000001FF 0x000001FF 0x 0x Heap L9 0xFFFFFF Virtual Page 0xFFFFFF Stack 0xFFFFFFFF 0xFFFFFFFF

9 Address Translation VPN PPN Virtual address Physical address Offset
Virtual page number (VPN) Physical Page Number (PPN) Translation table PAGE TABLE

10 … … What’s happening… Page Tables Disk Main Memory P1 - - - Processes
2 - 3 1 - 4 Processes - 1 - P1 P2 Pn P2 2 2 3 4 1 3 4 - 4 2 3 L9 1 3 Virtual page contents 4 - 2 Pn 3 2 4 -

11 Page Fault Situation where virtual address generated by processor is not available in main memory Detected on attempt to translate address Page Table entry is invalid Must be `handled’ by operating system Identify slot in main memory to be used Get page contents from secondary memory Part of disk can be used for this purpose Update page table entry Data can now be provided to the processor L9

12 Page Replacement Policies
Question: How does the page fault handler decide which main memory page to replace when there is a page fault? Principle of Locality of Reference A commonly seen program property If memory address A is referenced at time t, then it and its neigbhouring memory locations are likely to be referenced in the near future Suggests that a Least Recently Used (LRU) replacement policy would be advantageous temporal L9 spatial

13 Locality of Reference Based on your experience, why do you expect that programs will display locality of reference? Same address (temporal) Neighbours (spatial) Loop Function Sequential code Loop Program L9 Local Loop index Stepping through array Data

14 Page Replacement Policies
LRU might be too expensive to implement Alternatives Approximation to LRU First in First Out (FIFO) Random L9

15 Implementation of Address Translation
Easy, But can’t be done by OS, which is software Memory Management Unit (MMU): part of CPU; hardware that does address translation Problems: Big Page Table size Example: 32b virtual address, 16KB page size 256K virtual pages => MB page table size per process Has to be stored in memory, probably virtual address space Need multiple memory accesses for each memory access Translation Lookaside Buffer (TLB): memory in MMU that contains some page table entries that are likely to be needed soon TLB Miss: required entry not available = 218 L9

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